The present invention relates to ultraviolet radiation measuring means responsive to radiation in a selected ultraviolet band, and more particularly to a pyranometer for measuring ultraviolet-B in solar radiation.
Measurement of solar radiation may be made within particular bands with the ultraviolet spectrum, such as ultraviolet-A or ultraviolet-B. Measurement of solar ultraviolet-B radiation is a distinguishable endeavor from ultraviolet radiation measurement in general. Ultraviolet-B radiation, also referred to in the art as UV-B, is the radiation in the range of 280-320 nm. It is this portion of the UV spectrum that is associated with erythmal damage, for example sunburn. Prior art monitors for measuring outputs of ultraviolet lamps are, of course, well-known. They will have some utility in measuring ultraviolet radiation. However, the solar spectrum differs from that produced by common prior art electrical ultraviolet radiation sources.
It is for this reason that the present invention is characterized as a pyranometer rather than broadly as ultraviolet measuring apparatus. The "pyra" root refers to early forms of such apparatus in which heating on blackened metal strips due to solar radiation was measured. The present invention will provide for accuracy of UV-B measurement in ways not contemplated in the design of apparatus for measuring the output of artificial sources. The monitors for electrical sources do not address concerns uniquely associated with precise measurement of solar UV-B.
In accordance with the present invention, it is desired to provide a reference grade instrument useful for measurement of environmental UV-B, or other ultraviolet band such as UV-A. The need for measurement of UV-C with the present invention is not contemplated to be as great, Reference grade instruments are well-known in the art. However, as described below, there are significant applications of pyranometers that require their use in large numbers. Embodying such an instrument in a relatively low-cost form greatly increases its accessibility to users.
Measurement of solar UV-B is an important concern due to the scientific and environmental issues concerning depletion of the earth's stratospheric ozone layer. Stratospheric ozone attenuates UV-B. There is concern that people on the earth's surface will be exposed to undue risks of erythmal damage due to increased exposure to UV-B resulting from depletion of the ozone layer. Substantial depletions of the Antarctic stratospheric ozone layer was documented in J. C. Farman, B. G. Gardiner and J. D. Shanklin: Large losses of total ozone in Antarctica reveal seasonal ClO.sub.x /NO.sub.x interaction. Nature, 315(6016), 207-210, (1985). In M. Blumenthaler and W. Ambach: Measurements of the temperature coefficient of the Robertson-Berger sunburn meters and the Eppley UV Radiometer. Arch. Meteorol. Geophys. Bioclim., B36, 357-363, (1986), it is reported that UV-B flux has increased about 1% per year in the Swiss Alps. Consequently, there is a need for global monitoring of ultraviolet radiation levels in solar radiation reaching the earth's surface.
Measurement of UV-B flux due to solar radiation in urban areas is difficult due to presence of UV-B absorbers such as aerosols and pollution-caused ozone in the troposphere. Consequently, there is a need for establishment of an improved measuring technology and for an efficiently produced apparatus suitable for use in widespread UV-B monitoring networks. Existence of large-scale, geographically extensive networks is a prerequisite for establishing a complete and accurate UV-B climatology of the earth.
Measurement of solar UV-B can be done with primary spectroradiometers, which serve as reference instruments and provide baseline data. However, their expense and complexity makes them impractical for widespread use. Conventional secondary optical detection schemes, e.g. silicon detectors, are not really suited to this task since the amount of energy present in the ultraviolet-B spectrum is only a small fraction of the total amount of energy available in the visible portion of the spectrum. Rejection of response to visible portions of the spectrum is very important. Even if a detector converts only a very small percentage of visible radiation into a signal supposedly indicative of UV-B, its utility for indicating slight increases in UV-B flux over a period of time will be totally compromised.
Current ultraviolet-B radiation detectors are further embodiments of a scheme for detecting UV-B described in D. F. Robertson: Solar ultraviolet radiation in relation to human sunburn and skin cancer. Ph.D. thesis, University of Queensland, Australia, (1972). This basic for of measurement was improved by Berger, as described in, D.S. Berger: The sunburning ultraviolet meter: design and performance. Photochem. Photobiol., 24, 587-593 (1976). Present versions of these instruments are often referred to as Robertson-Berger meters, or R-B meters.
In the general scheme of an R-B meter, radiation is directed to a phosphor which is excited by UV-B and re-emits at another wavelength. In this manner, a detector can be used which is nominally insensitive to red visible radiation, and response to UV-B is provided while visible radiation is desired to be rejected. Robertson uses a photomultiplier to sense radiation intensity emitted by the phosphor. An updated version using a solid state radiation detector rather than a photomultiplier is disclosed in U.S. Pat. No. 4,348,664 issued Sep. 7, 1982 and assigned to Elder pharmaceuticals. While this system is nominally insensitive to red radiation, it is utilized to measure the output of UV lamps to assure uniform dosage therefrom. However, this apparatus is not intended for use with the sun as a source. It would be imprecise for the reasons discussed above if used in the context of solar radiation. Gaseous ultraviolet lamps have a significantly lower "red" component in their output spectrum than solar radiation. Even a small fraction of the visible component of sunlight can contain more power than the UV-B component. Consequently, precision can be destroyed if even a small portion of the sunlight is not rejected. The apparatus as disclosed in the patent nominally rejects "red" response from an artificial source, but may not be operative to reject "red" at the levels received from solar radiation.
It is important to be able to provide a pyranometer for solar UV-B measurement since there is a need to perform solar measurement that is not addressed by instruments designed to monitor the output of UV lamps, and because it requires only a limited degree of imprecision in the output of the pyranometer to render it inoperative for meeting the objectives of the present invention.